Lost foam casting is a casting process that combines features of both investment casting and sand casting. It is a type of evaporative pattern casting where the pattern is made of foam instead of wax. Similar to investment casting, the foam pattern is coated with a refractory material. However, instead of forming a solid mould, it is surrounded by compacted, unbonded sand for structural support. When molten metal is poured into the mould, the foam pattern vaporises, leaving behind a cavity that is filled with metal. This process integrates the precision and quality of investment casting with the cost-effectiveness and flexibility of sand casting.

Lost Foam Casting Process:

1. A foam pattern (as shown in the images above) is created using expanded polystyrene (2.5% polystyrene, 97.5% air). This can be shaped through closed-die moulding, machining, or assembly from multiple components. Risers and gates are incorporated at this stage.
2. Multiple finished patterns, including the gating system, are glued together to form a cluster.
3. The cluster is coated with a permeable refractory slurry, typically by dipping. Once dry, the refractory coating forms a rigid shell around the foam pattern. The type of refractory material used depends on the required surface finish.
4. The cluster is then placed inside a foundry flask or box and surrounded by loose, unbonded sand, which is vibrated to ensure proper compaction.
5. Molten metal is poured into the mould, causing the foam pattern to vaporise instantly, leaving behind a metal casting within the refractory shell.
6. After the metal cools and solidifies, the sand and refractory shell are broken away. The risers and gates are removed, leaving behind the final casting.
7. The cast component can then undergo finishing, treatment, or machining as required.

Advantages of Lost Foam Casting:

Lost foam casting offers several benefits, including:

● High dimensional accuracy with typical linear tolerances of ±0.005 mm/mm.

● Excellent surface finish, typically between 2.5 and 25 µm.

● No requirement for draft angles to facilitate mould removal.

● Absence of parting lines and flash, ensuring a smoother and more consistent surface finish without additional machining.

● Use of unbonded sand, which is simpler and more economical than greensand or resin-bonded sand.

● Fewer processing steps compared to investment casting, reducing overall costs.

● Minimal use of risers, leading to lower metal consumption and reduced finishing requirements.

● Natural directional solidification, resulting in predictable casting quality with fewer defects.

● Foam patterns are easy to shape, carve, glue, and handle.

● Capability to consolidate multiple components into a single complex casting, minimising post-casting assembly.

● Suitable for a wide range of materials, including aluminium and nickel alloys, steels, cast irons, stainless steels, and copper alloys.

● Versatility in casting size, ranging from 0.5 kg to several tonnes.

● Ability to produce thin walls as small as 2.5 mm, with no upper limit.

Disadvantages of Lost Foam Casting:

Despite its advantages, lost foam casting has some limitations, including:

● High pattern costs for low-volume production.

● Foam patterns, while lightweight and easy to handle, are fragile and prone to damage or distortion.

● If closed-die moulding is used to create patterns, the cost of the die can be significantly high.